1. Field
Provided are a positive active material for a lithium secondary battery and a method of manufacturing the same.
2. Description of the Related Technology
Electronic equipment has recently become down-sized and lighter as portable electronic devices have become more widely used. Accordingly, research is conducted on lithium secondary batteries with high energy density as power sources for portable electronic devices.
A lithium secondary battery typically includes materials that reversibly intercalate or deintercalate lithium ions as negative and positive electrodes and also includes an organic electrolyte or a polymer electrolyte through which lithium ions move between the positive and negative electrodes. Electrical energy is produced through oxidation/reduction when lithium ions are intercalated/deintercalated at the positive and negative electrodes.
A lithium metal has been typically used as the negative active material for such a lithium secondary battery. However, since the lithium metal can cause battery short-circuiting due to dendrites during charging and discharging, a lithium secondary battery may explode. In order to solve this problem, a carbon-based material that can reversibly accept or supply lithium ions, while maintain structural and electrical characteristic, and that has a similar half-cell potential to a lithium metal during the intercalation and deintercalation of lithium ions is widely used as a negative active material.
On the other hand, a positive active material for a lithium secondary battery may include the chalcogenide compound of a metal that can intercalate/deintercalate lithium ions and practically, a composite metal oxide such as LiCoO2, LiMn2O4, LiNiO2, LiNi1−xCoxO2 (0<x<1), LiMnO2, and the like. Among these positive active materials, LiNiO2 has a large charge capacity but is hard to synthesize and has poor cycleability. Mn-based active materials such as LiMn2O4, LiMnO2, and the like are easy to synthesize and environment-friendly as well as cost relatively low but has small capacity. In addition, since LiCoO2 has electric conductivity ranging from 10−2 to 1 S/cm at room temperatures, high operating battery voltage, and excellent electrode characteristic, it is widely used. However, LiCoO2 has the problem of low stability during the high rate charge and discharge and cannot be efficiently operated at high temperature and voltages over 4.3V.
In general, these composite metal oxides are prepared by mixing raw solid powders and firing the mixture in a solid-phase reaction method. For example, Japanese Patent Notification Hei 8-153513 discloses a method of preparing LiNi1−xCoxO2 (0<x<1) by mixing Ni(OH)2 with Co(OH)2 or hydroxides including Ni and Co, heat-treating the mixture, and pulverizing. Otherwise, a complete crystalline active material may be prepared by reacting LiOH, an Ni oxide, and a Co oxide, primarily heat-treating the mixture at 400 to 580° C. to form an initial oxide, and secondarily heat-treating the initial oxide at 600 to 780° C.
When an active material prepared according to the above method is applied to a lithium secondary battery, it may have a side reaction with an electrolyte and thus, may not maintain the main characteristics of the lithium secondary battery at high voltages.